The present invention relates generally to a thermal energy transfer device for use with an x-ray generating device and, more specifically, to a thermal gradient device for use with an x-ray tube.
Typically, an x-ray generating device, referred to as an x-ray tube, includes opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is commonly fabricated from glass or metal, such as stainless steel, copper, or a copper alloy. The electrodes include a cathode assembly positioned at some distance from the target track of a rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode assembly may be stationary. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or a tungsten alloy. Further, to accelerate electrons used to generate x-rays, a voltage difference of about 60 kV to about 140 kV is commonly maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode assembly at high velocity. A small fraction of the kinetic energy of the electrons is converted to high-energy electromagnetic radiation, or x-rays, while the balance is contained in back-scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from a focal spot, and may be directed out of the vacuum vessel along a focal alignment path. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the vacuum vessel to allow an x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed along the focal alignment path to penetrate an object, such as a human anatomical part for medical examination and diagnostic purposes. The x-rays transmitted through the object are intercepted by a detector or film, and an image of the internal anatomy of the object is formed. Likewise, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at an airport.
Since the production of x-rays in a medical diagnostic x-ray tube is by its very nature an inefficient process, the components in an x-ray tube operate at elevated temperatures. For example, the temperature of the anode""s focal spot may run as high as about 2,700 degrees C., while the temperature in other parts of the anode may run as high as about 1,800 degrees C. The thermal energy generated during tube operation is typically transferred from the anode, and other components, to the vacuum vessel. The vacuum vessel, in turn, is generally enclosed in a casing filled with a circulating cooling fluid, such as dielectric oil, that removes the thermal energy from the x-ray tube. Alternatively, in mammography applications, for example, the vacuum vessel, which is not contained within a casing, may be cooled directly with air. The casing, when used, also supports and protects the x-ray tube and provides a structure for mounting the tube. Additionally, the casing is commonly lined with lead to shield stray radiation.
As discussed above, the primary electron beam generated by the cathode of an x-ray tube deposits a large heat load in the anode target and rotor assembly. In fact, the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, the balance being converted to thermal energy. This thermal energy from the hot target is conducted and radiated to other components within the vacuum vessel. The fluid circulating around the exterior of the vacuum vessel transfers some of this thermal energy out of the system. However, the high temperatures caused by this thermal energy subject the x-ray tube components to high thermal stresses that are problematic in the operation and reliability of the x-ray tube. This is true for a number of reasons. First, the exposure of components in the x-ray tube to cyclic high temperatures may decrease the life and reliability of the components. In particular, the anode assembly is subject to thermal growth and target burst. The anode assembly also typically includes a shaft that is rotatably supported by a bearing assembly. This bearing assembly is very sensitive to high heat loads. Overheating of the bearing assembly may lead to increased friction, increased noise, and to the ultimate failure of the bearing assembly. This problem is especially acute for mammography systems as a result of the high impact temperatures and tight acoustic noise requirements involved. Due to the high temperatures present, the balls of the bearing assembly are typically coated with a solid lubricant. A preferred lubricant is lead, however, lead has a low melting point and is typically not used in a bearing assembly exposed to operating temperatures above about 330 degrees C. Because of this temperature limit, an x-ray tube with a bearing assembly including a lead lubricant is limited to shorter, less powerful x-ray exposures. Above about 400 degrees C., silver is generally the lubricant of choice, allowing for longer, more powerful x-ray exposures. Silver, however, increases the noise generated by the bearing assembly. Ideally, if the operating temperature of the bearings could be sufficiently reduced, vacuum grease could be used to lubricate the bearings, decreasing noise and increasing rotor speed and bearing life.
The high temperatures encountered within an x-ray tube also reduce the scanning performance or throughput of the tube, which is a function of the maximum operating temperature, and specifically the anode target and bearing temperatures, of the tube. As discussed above, the maximum operating temperature of an x-ray tube is a function of the power and length of x-ray exposure, as well as the time between x-ray exposures. Typically, an x-ray tube is designed to operate at a certain maximum temperature, corresponding to a certain heat capacity and a certain heat dissipation capability for the components within the tube. These limits are generally established with current x-ray routines in mind. However, new routines are continually being developed, routines that may push the limits of existing x-ray tube capabilities. Techniques utilizing higher instantaneous power, longer x-ray exposures, and increased patient throughput are in demand to provide better images and greater patient care. Thus, there is a need to remove as much heat as possible from existing x-ray tubes, as quickly as possible, in order to increase x-ray exposure power and duration before reaching tube operational limits.
The prior art has primarily relied upon removing thermal energy from the x-ray tube through the cooling fluid circulating around the vacuum vessel. It has also relied upon increasing the diameter and mass of the anode target in order to increase the heat storage capability and radiating surface area of the target. These approaches have been marginally effective, however, they are limited. The cooling fluid methods, for example, are not adequate when the anode end of the x-ray tube cannot be sufficiently exposed to the circulating fluid. Likewise, the target modification methods are generally not adequate as the potential diameter of the anode target is ultimately limited by space constraints on the scanning system. Further, a finite amount of time is required for heat to be conducted from the target track, where the electron beam actually hits the anode target, to other regions of the target.
Therefore, what is needed are devices providing cooler running x-ray tube bearings, allowing lubricants such as vacuum grease to be used. This would reduce bearing noise and allow higher rotor speeds to be achieved. Higher rotor speeds would, in turn, greatly reduce the impact temperature of the x-ray tube target created by the electron beam, increasing the operating life of the x-ray tube.
The present invention overcomes the aforementioned problems and permits greater x-ray tube throughput by providing cooler running bearings with higher steady state power capability.
In one embodiment, an x-ray generating device for generating x-rays includes a vacuum vessel having an inner surface forming a vacuum chamber; an anode assembly disposed within the vacuum chamber, the anode assembly including a target; a cathode assembly disposed within the vacuum chamber at a distance from the anode assembly, the cathode assembly configured to emit electrons that strike the target of the anode assembly, producing x-rays and residual energy in the form of heat; a shaft coupled to the vacuum vessel by a bearing assembly, the shaft having a first end and a second end, the first end of the shaft having a support for supporting the target; a thermal gradient device positioned adjacent to and in thermal communication with the second end of the shaft, the thermal gradient device operable for transferring heat away from the second end of the shaft; and a fin structure positioned adjacent to and in thermal communication with the thermal gradient device, the fin structure operable for convectively cooling the thermal gradient device.
In another embodiment, a thermal energy transfer device for use with an x-ray generating device, including an anode assembly having a target, a cathode assembly at a distance from the anode assembly configured to emit electrons that strike the target, producing x-rays and residual energy in the form of heat, and a rotatable shaft supported by a bearing assembly, includes a thermal gradient device positioned adjacent to and in thermal communication with one end of the shaft, the thermal gradient device operable for transferring heat away from that end of the shaft, and a fin structure positioned adjacent to and in thermal communication with the thermal gradient device, the fin structure operable for convectively cooling the thermal gradient device.
In a further embodiment, an x-ray system includes a vacuum vessel having an inner surface forming a vacuum chamber; an electron source disposed within the vacuum chamber, the electron source operable for emitting electrons; an x-ray source disposed within the vacuum chamber, the x-ray source operable for receiving electrons emitted by the electron source, producing x-rays and residual energy in the form of heat; a shaft coupled to the vacuum vessel by a bearing assembly, the shaft having a first end and a second end, the first end of the shaft having a support for supporting the x-ray source; and a thermal energy transfer device positioned adjacent to and in thermal communication with the second end of the shaft, the thermal energy transfer device operable for transferring heat away from the second end of the shaft.